In a CANDU reactor, the fuel and coolant are separated from the moderator by a horizontal fuel channel. The fuel channel consists of a pressure tube and calandria tube separated by a gas-filled annulus. Within the annulus there are a number of circular springs, known as garter springs, spaced longitudinally for maintaining separation of the pressure tube from the calandria tube.
During a postulated loss of coolant accident with coincident impaired emergency core cooling, pressure tubes become hot and may deform into contact with its surrounding calandria tube. If the pressure tube contacts the calandria tube, the temperature of the calandria tube rises and can lead to film boiling on the outside of the calandria tube. Film boiling or dryout is the condition where water is prevented from contacting the surface of the calandria tube by a film of steam. When this occurs, the ability of the moderator to remove heat from the fuel channel is severly reduced. As a consequence, the temperature of the calandrial tube increases. In extreme cases the integrity of the fuel channel is endangered.
Film boiling could be prevented by lowering the moderator temperature, i.e. increasing moderator subcooling. This would be costly since larger moderator heat exchangers would be required to maintain lower moderator temperatures. Another method of preventing the calandria tube from going into dryout is to reduce the contact conductance between the pressure and calandrial tubes. One approach proposed for limiting thermal conductance rate was to incorporate surface roughness to one of the adjacent tube surfaces, for example, by shot peening or knurling, in order to limit the contact area. Initial findings from this work suggest that shot peening does not generate enough surface roughness to significantly alter the intertube conductance. Furthermore, this approach is not entirely satisfactory as it is difficult to implement and the structural properties of the treated tube are compromised.